1. Technical Field of the Invention
The present invention relates to pressure mapping and, more particularly, to a method, apparatus and system of lifetime decay measurement pressure mapping.
2. Description of Related Art
Aerodynamic test data of surfaces, such as airfoils, are important for improving aerodynamic loading and for optimization of aerodynamic efficiency. Since, aerodynamic forces acting on an airfoil results largely from the distribution of pressure over the surface, airfoils and other aircraft components are commonly tested in wind tunnels to gather test data for use in verification of characteristics and in design improvements. It is common practice to measure surface pressures using a pressure model in a wind tunnel and compute the force distributions from the pressure data collected.
A number of techniques have been developed over the years to collect pressure measurements. A current technique is to build a pressure model having hundreds of small orifices or pressure taps machined into their surfaces. These small orifices must be drilled precisely normal to the surface with tight tolerances on location and diameter. Additionally, each pressure orifice must be connected to a pressure transducer device. The pressure taps communicate at one end with the pressure at the surface of the structure and at the other end with the pressure transducer.
Since there are usually too many orifices to leave room for the transducers inside the model, hundreds of thin tubes must be used to establish connection to the internally located transducers. Because of the spatial requirements, the number of sensors that can be used in this type of system is limited and, thus, the resolution of the measurements is also limited. Moreover, overall forces and moments cannot always be measured accurately with the above-mentioned technique, because of the poor spacial resolution. Lastly, utilizing pressure models with pressure taps is time consuming and expensive to build.
In another approach, mechanical or electrical pressure sensors can be affixed to the external surface of the pressure model to be tested. However, this approach exhibits many of the same shortcomings as the above-mentioned approach. The sensors are generally relatively large compared to the pressure model which limits the spacial resolution of data that can be gathered during the wind tunnel testing. Additionally, such sensors ordinarily extend above the surface of the model into the air stream which tends to alter the air stream and alter the measured values.
The field of pressure sensitive paint barometry has been developed in response to the difficulties arising from the above-described conventional techniques. Pressure sensitive paint barometry has developed as a result of the discovery that a surface may be coated with materials which are capable of being excited by light and which will emit light as a function of pressure which may be used as a measure of pressure on the surface. The coated material is made of an adhesive and an active agent, for example, that emits light when excited by radiation of a particular type, such as ultraviolet or visible blue light. For some types of active agents, the presence of oxygen, such as found in the air, quenches or reduces the light emission. It has been found that light output of some active agents is inversely proportional to the partial pressure of the oxygen. Therefore, the higher the pressure of oxygen in the atmosphere contacting the pressure sensitive paint, the lower the light emission of the paint.
Other pressure sensitive paints, when flashed with light, exhibit a returned or reflected light with a decay time constant which is a function of surface pressure disposed on a painted surface. However, these lifetime decay measurement systems require a plurality of exposures to capture enough light decay data to accurately compute surface pressure of a wind tunnel model.
Current methods requires that system components be manually set-up for each test image to capture exposures with different timing. Each exposure or test image requires programming a camera shutter for a specific instant of time subsequent to light exposure. Since wind tunnel test time is very expensive costing up to $10,000 per hour not including labor cost, extended manual setup time for multiple camera exposures is very expensive. The multiple programming and set-up is not only time consuming but also introduces inconsistent results. Current methods introduce as much as thirty percent error because of inadequate synchronization of a few microseconds between the camera equipment and the lighting equipment.
The present invention achieves technical advantages as an apparatus, system and method of determining surface pressure of a surface which is coated with a pressure sensitive paint which, when flashed with light, exhibits return light with a decay time constant as a function of surface pressure. The system controls a light source which is adapted to emit a train of light pulses directed at the coated surface and a camera adapted to collect light level data from the surface. Further, a synchronizer is used to provide timing and control of the light source and the camera in which the synchronizer enables the light source and subsequently enables the camera to collect light level data in a first gate period and a second gate period. The first gate period is set to a predetermined time period following a light pulse and the second gate period is set to a predetermined time period subsequent to the first gate period. A decay time constant is determined from the data collected in the first gate period and the second gate period.
Although the new advantages of the present invention are too numerous to discuss fully here, several advantages are of particular interest. First, the present invention synchronizes the electronic shutter of a digital camera to provide defined exposures with consistent flash numbers providing for the determination of more precise decay time constants. Second, the present invention also provides for higher productivity eliminating the need for operator set-up programming for each camera exposure, thus, allowing an operator to collect up to 10-20 times as many exposures as current methods, given the same amount of time. Additionally, the present invention offers an operator a high level of confidence that the equipment is operating properly by observing a proper sequence of events.